This invention relates to an apparatus and method for uniformly processing a substrate and reducing particulate contamination of the substrate, the invention being particularly useful in plasma processes.
In semiconductor fabrication processes, process gases are used to deposit material upon, etch, or clean semiconductor substrates. Semiconductor processes are typically carried out in an enclosed process chamber having gas distributors for distributing process gas in the chamber. One problem with conventional process chambers is their relative inability to achieve a uniform distribution or concentration of reactive process gas species across the substrate surface, particularly at the peripheral edge of the substrate. The distribution of reactive process gas species across the substrate affects the uniformity and rate of processing of the substrate. For example, a process chamber 10 having a single process gas distributor 11a positioned directly above the substrate 12, as shown in FIG. 1, causes an asymmetric distribution of reactive process gas across the substrate 12, with the concentration of reactive process gas highest at the center 13 of the substrate and lowest at the peripheral edge 14 of the substrate. Conversely, a process gas distributor 11b positioned around the peripheral edge 14 of the substrate causes the concentration of reactive process gas to be highest at the peripheral edge 14 of the substrate and lowest at the center 13 of the substrate. The concentration of reactive process gas across the substrate 12 can also vary when the flow of process gas across the surface of the substrate is unrestrained, such as for example, when the process gas can freely flow around the peripheral edge 14 of the substrate 12.
The concentration gradient of reactive process gas across the substrate 12 can be reduced using process gas containment structures to contain the flow of process gas on the substrate 12. For example in plasma processes, it is known to use a focus ring 15 around the substrate to maintain a more uniform distribution of the reactive process gas on the surface of the substrate 12. However, one problem with conventional plasma focus rings is that the process gas stagnates at the portion 16 of the focus ring 15 adjacent to the peripheral edge 14 of the substrate 12 to form contaminant particles 17 that flake off and contaminant the substrate 12, as shown in FIG. 1. It is difficult to non-destructively detect the contamination during the processing of the substrate, and it is only in the final processing stages when the fully processed semiconductor substrates are worth between $50,000 to $100,000 each, that the contaminants are discovered, and the entire substrate is rejected.
Several conventional methods are used to reduce particulate contamination of the substrate. In one method, the process chamber and the components therein are periodically cleaned to remove the deposits formed on the components using plasmas of corrosive gases, such as NF.sub.3 plasmas. However, the corrosive plasmas erode the processing components and limit the life of the process chamber. Also, the cleaning process step increases the down time of the apparatus and reduces process throughput.
Another solution to the particulate contamination problem taught by U.S. Pat. No. 5,423,918 to Gupta, et al., involves a final processing step of raising the focus ring above the substrate and flowing inert gas in the chamber to flush out the contaminant particulates accumulated in this region. However, mechanical stresses resulting from raising and lowering of the focus ring can dislodge the deposits formed on the focus ring to form additional contaminant particulates in the process chamber. Also, the steps of raising and lowering the focus ring lengthens overall processing time and reduces process efficiency.
Thus, there is a need for a substrate processing apparatus capable of maintaining a substantially uniform concentration of reactive process gas across the substrate surface, and in particular, at the peripheral edge of the substrate. There is also a need for an apparatus and method that reduces formation of particulate contaminants in the process chamber.